1. Field of the Invention
The invention relates to a protection circuit for protecting the thyristors (SCR'S) of a series circuit of several thyristors, and, in particular, thyristors each of which has a corresponding firing circuit to which its anode-cathode voltage is applied.
2. Description of the Prior Art
Such a protection circuit for serial connection of thyristors forming a valve is known from the German Auslegeschrift DT-AS No. 20 05 724. The aforesaid known protection circuit comprises an arrangement wherein the polarity of the voltage across the valve is sensed and a first signal is delivered via a multivibrator circuit to an AND gate for a predetermined monitoring period corresponding to the maximum recovery time of the thyristors, if the polarity of the voltage, after a commutation, becomes negative, and a second signal is delivered directly to such AND gate if the polarity of the voltage becomes positive. Thus, a firing signal is delivered to all thyristors of the valve via the AND gate only if the second signal is delivered within the predetermined monitoring time period.
In the aforesaid known protection circuit, the entire series circuit of thyristors is monitored by employing a voltage divider to derive a signal related to the polarity of the total voltage present across the series-connected thyristors. In such an overall monitoring system, the different properties of the individual thyristors are not taken into account, as the voltage waveform across the entire series circuit is not an indication of the voltage waveforms at the individual thyristors. However, because of the differing properties of the thyristors, the voltage waveforms at the individual thyristors can be different from each other and from the total voltage across their series connection. Thus, it is conceivable, that the individual voltage waveforms may equalize one another so that the total voltage monitored by the protection circuit indicates protective measures need not be taken, while, in fact, one of the thyristors may be in danger of being overstressed. It is a further disadvantage of the aforesaid known protection circuit that in the event of a disturbance, all of the respective thyristors in the series circuit are fired. This requires an unnecessarily large amount of firing power, which must be derived from the anode-cathode voltage of the respective thyristors.
It is therefore an object of the present invention to provide a protection circuit arrangement for protecting the thyristors in a series circuit of several thyristors which takes the properties of each individual thyristor into account and which during operation is insulated against interference.